The her2 gene is overexpressed in approximately 30% of metastatic breast cancers, and is associated with rapid disease progression and reduced overall survival. The median duration of response to the HER2-targeted drug Herceptin is less than one year, indicating that acquired drug resistance is a major clinical problem in the treatment of HER2-overexpressing metastatic breast cancer. The long-term goal of this application is to identify mechanisms and predictors of Herceptin resistance in order to improve the survival of patients with HER2-overexpressing breast cancer. Herceptin-resistant cells express reduced levels of the cyclin-dependent kinase (cdk) inhibitor p27 and show a unique receptor cross-talk between insulin-like growth factor-I receptor (IGF-IR), HER2, and HER3. Our central hypothesis is that the IGF-IR/HER2/HER3 complex activates downstream kinase signaling pathways that promote phosphorylation and degradation of p27, causing increased proliferation of HER2-overexpressing breast cancer cells. Using a particularly innovative multidisciplinary approach that combines nanotechnology, genetics, and pharmacology, we will determine (1) the mechanisms by which p27 is down- regulated in acquired Herceptin resistance, (2) the role of the IGF-IR/HER2/HER3 receptor complex in acquired Herceptin resistance, and (3) if IGF-IR, HER3, and FAK are in vivo targets for improving response to Herceptin. Access to multiple models of acquired Herceptin resistance and multiple patient tumor tissue sets places us in a unique position to discover novel therapeutic targets and markers of resistance. Ultimately, this study will benefit human health by identifying new molecular targets, novel drug combinations, and molecular markers of drug resistance in HER2-overexpressing breast cancer. Understanding the mechanisms leading to acquired Herceptin resistance will ultimately lead to refined therapeutic strategies and improved survival rates for patients with breast cancer.